Compound fenestration units, commonly referred to as mulled fenestration units, are formed by joining two or more individual window or door units, which will hereinafter be referred to as component units, so as to form a combination of windows, or windows and doors, that can be handled and installed as a single unit, and which give the appearance of being a single unit. A simple system for joining the component units involves the placing of spacer boards between the units to be joined and installing screws or other fasteners through the frames of the component units, into the spacer boards, to join the units. Other systems for joining the units involve the use of interlocking brackets or other like devices that can be separately installed on the facing surfaces of the frames to be joined and then coupled together to form the compound unit.
An important aspect of compound fenestration units is that a great variety of different compound fenestration units can be formed from a relatively limited set of component units. Assembly of component window or door units into compound fenestration units involves not only mechanical coupling of the component window units, but also sealing of the joints between the component units against rain, wind, and other intrusions. Additionally, it is preferred that any sealing system accommodate a variety of gap arrangements and provide a suitable appearance to the compound unit. Silicone RTV, for example, can provide effective sealing for virtually any gap arrangement, either by itself or in combination with weather stripping or other covering or trim pieces, but the appearance of the sealed unit may be less than desirable, and may not provide the desired appearance of a single integrated unit. Additionally, the skill and equipment needed for the proper application of silicone or other like sealants may not always be readily available in all manufacturing settings. More visually pleasing sealing methods, such as preformed gaskets or trim materials can suffer, from a lack of adaptability to different combinations of component window units. There thus is a continuing need for a method and apparatus for joining together individual window units or door units to form multi-unit fenestration assemblies that addresses the problems and shortcomings of the prior art. It is to the provision of such that the present invention is primarily directed.